The present invention relates to the production of petroleum and in particular to the control of beam pumping units used for producing petroleum. In particular, the invention pertains to the control of beam pumping units wherein a pump located at the bottom of the well is actuated by a string of steel sucker rods that are reciprocated by a beam pumping unit at the surface. In this type of pumping unit, it is desirable to shutdown the unit when the pump barrel does not completely fill with liquid on the upstroke of the pump. When the pump barrel does not completely fill with liquid on the upstroke, on the succeeding downstroke, the pump plunger will travel some distance before it contacts the liquid in the pump barrel. This will produce pounding in the pump with severe mechanical stresses and vibrations. The mechanical vibration and stresses lead to premature pump failures and excessive maintenance of the rods and pumping units. Thus, various systems such as those described in the above patent have been designed to determine when the pump barrel has failed to completely fill with liquid. If the pump barrel does not completely fill with liquid on the upstroke, the well is said to have "pumped-off" and the well should be shut-in until sufficient liquid has drained into the well.
As described in the co-pending application, the time when a well has pumped-off may be determined by measuring the energy input to the top of the rod string. The energy input is determined by measuring the load on the rod and the displacement of the rod and integrating the product of the load times displacement. A plot of load versus displacement provides a surface dynamometer card whose area is equal to the total energy input during one stroke of the pump. When the well is pumped-off the energy input to the rod is reduced since the load on the pump plunger on the initial part of the downstroke remains high as a result of the column of oil above the traveling valve and the lack of oil in the pump barrel. The net effect is a reduction in the energy input to the well that can be used to signal the pumping-off of the well.
While the system described in the co-pending application has been used successfully in many cases, the difference between the energy input for a pumping condition and the energy input for a pumped-off condition is usually only 5-15% of the total power input. Thus, errors in the measurement of the load on the rod string or the displacement of the rod string can produce an error in the final result which may substantially equal the difference in energy input between the pumped-off condition and the pumping condition.